Augmentors, also referred to as afterburners, are located in the exhaust section of many jet engines. Augmentors are designed to inject fuel directly into the engine exhaust. The injected fuel, when ignited provides supplemental thrust. Since augmentors are normally used to provide supplemental thrust in emergency situations, it is desirable to inject fuel into the exhaust immediately after receiving the augmentor command. Therefore, it is desirable to leave the augmentor manifolds full of fuel when the augmentor nozzles are closed. Unfortunately, since the augmentor is located in the exhaust section of the engine, it gets extremely hot and fuel in the manifold will tend to evaporate.
When jet fuel evaporates, it can leave layers of carbon on the inside of the augmentor. This carbon buildup can damage and may eventually block the nozzles. Evaporation can be avoided by continuously circulating fuel through the augmentor manifold.
When an augmentor is spraying fuel into the exhaust, it is said to be in the active mode. When an augmentor is not spraying fuel into the exhaust, it is said to be in the dry mode. Thus, fuel being recirculated to prevent evaporation is referred to as dry flow.
In one known arrangement (see U.S. Pat. No. 4,802,334), a system is described wherein the augmentor is drained after each use to avoid carbon buildup (also known as coking) due to fuel evaporation. When fuel is desired, it is pumped at a first, high rate, to fill the augmentor manifold and at a second, lower rate when the augmentor manifold is full.
In other known arrangements, recirculating flow may be provided by using a valve in the return path which is closed when the augmentor demands fuel. This return path valve could be a simple shutoff valve or, alternatively, a flow fuse or vent valve designed to close when the fuel pressure in the augmentor reaches a predetermined level.
Augmentor nozzles spray fuel into the engine exhaust in a predetermined pattern. In order to limit the number of control lines and the amount of control logic, it is desirable to use pressure actuated valves in the augmentor nozzles. The pressure actuated valves open when the fuel pressure in the actuator reaches a predetermined level. As an example, valves known as cracking valves may be used. Cracking valves are forced open by the pressure of the fuel in the augmentor manifold.
It would, therefore, be desirable to design an augmentor fuel supply system which achieves the pressures necessary to open the pressure actuated augmentor nozzles during augmentor operation, while providing for a continuous flow of fuel through the augmentor manifold during dry operation.